Generally, an electrochromic device is capable of modulating the optical properties (transmittance and/or reflectance and/or absorbance) when an electric field is applied thereacross. More specifically, it comprises at least one active electrochromic electrode capable of reversibly changing optical state on application of an electric charge.
A conventional electrochromic device generally comprises a substrate (1) on which are successively deposited (FIG. 1):                a first current collector (2);        a first electrochromic electrode (3);        an electrolyte (4);        a second electrochromic electrode (5); and        a second current collector (6).        
As already indicated, at least one of these electrochromic electrodes is optically active, which enables to modulate the optical properties of an electromagnetic radiation (7).
The electrochromic materials used to form an optically-active electrode may be organic, inorganic, or hybrid. Their nature allows a change of optical state, either by cation insertion, or by cation extraction.
Such an optical state change is obtained by application of an electric field across the electrochromic device.
As already indicated, electrochromic devices comprise two electrochromic electrodes. Typically, the second electrochromic electrode enables to store cations. It may be transparent, whatever the cation flow. It may also act as a complementary electrode and have an optical state (transparent, colored . . . ) identical to that of the first electrode but with an inverse cation flow.
Electrochromic devices having this second type of configuration (complementary electrode) are generally preferred, given that they improve the optical perception of the change of optical state (contrast). For example, the first electrode may be made of a WO3 material while the second electrode may be made of NiO. This couple of materials allows the following electrochemical reactions:WO3+xLi+ (transparent)← →LixWO3 (colored)LixNiO (transparent)← →NiO+xLi+(colored)
The performance of electrochromic devices is particularly assessed by means of the following indicators:                contrast: the difference between the maximum and the minimum of the optical response of the device, expressed in terms of percentage between two values, often of transmission or reflection. The higher the contrast, the more effective the device is considered.        optical density: the quantity of charges to be brought to the system to switch, that is, to pass from the minimum state to the maximum state or conversely. This corresponds to the efficiency of the transformation of the optical behavior by the quantity of injected charges. For a given charge, the greater the optical transformation, the more effective the device is considered.        switching time: the time necessary for the device to ensure the passing from one optical state to another, which is fixed for a given contrast. The shorter the switching time, the more effective the device is considered.        
Typically, the switching time is one of the major limitations of prior art electrochromic devices. It may generally vary from a few tens of seconds to a few minutes according to the architecture of the device and to the materials used.
All-solid electrochromic devices operating by insertion of cations (Li+ for example) within inorganic materials may have relatively long switching times. Indeed, Li+ cations are less mobile than protons. Further, an all-solid electrolyte has a lower ion conductivity than a liquid electrolyte. Accordingly, the cation migration kinetics is slower, which lengthens the switching time.
As an example, document U.S. Pat. No. 7,265,890 describes an all-solid inorganic electrochromic device operating by insertion of Li+ cations in the infrared range. It comprises a first electrochromic electrode, an electrolyte, and a second transparent electrochromic electrode behaving as an Li+ cation storage electrode.
Such an electrochromic device is typical of prior art configurations. The second electrochromic electrode is a transparent cation storage electrode, which is thus optically passive. Generally, it has a cation storage capacity larger than that of the first electrochromic electrode forming the active electrode.
Typically, the quantity of cations (Li+ for example) injected into the storage electrode corresponds to the maximum capacity of cations which can be reversibly inserted into the available thickness and de-inserted therefrom.
The saturation of the storage electrode enables to compensate for a possible loss of cations during the cycling (insertion irreversibility). It further enables to improve the chemical stability of the device by anticipating the possible oxidation of part of the cations, which might cause a failure of the device.
Anyhow, the cation switching time in such electrochromic devices is not satisfactory The present invention aims at solving this technical problem for any type of electrochromic device and, in particular, in all-solid inorganic electrochromic devices.